This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In a typical cellular radio system, user equipments (UEs) can communicate via a radio access network (RAN) to one or more core networks (CN). The RAN generally covers a geographical area which is divided into radio cell areas. Each radio cell area can be served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A radio cell is a geographical area where radio coverage is generally provided by the radio base station at a base station site. Each radio cell can be identified by an identity within the local radio area, which is broadcast in the radio cell. The base stations communicate over the air interface operating on radio frequencies with the UEs within range of the base stations. In some radio access networks, several base stations may be connected (for example, by landlines or microwave) to a radio network controller (RNC) or a base station controller (BSC). The radio network controller may be configured to supervise and coordinate the various activities of the plurality of base stations connected thereto. The radio network controllers may also be connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM). The Universal Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using Wideband Code Division Multiple Access (WCDMA) for UEs. As an alternative to WCDMA, Time Division Synchronous Code Division Multiple Access (TD-SCDMA) could be used. In a standardization forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate e.g. enhanced data rate and radio capacity. The 3GPP has undertaken to evolve the UTRAN and GSM based radio access network technologies. The first releases for the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) specification have been issued. The Evolved Universal Terrestrial Radio Access Network (E-UTRAN) comprises the Long Term Evolution (LTE) and System Architecture Evolution (SAE). Long Term Evolution (LTE) is a variant of a 3 GPP radio access technology where the radio base station nodes are connected to a core network (e.g., via Access Gateways (AGWs)) rather than to radio network controller (RNC) nodes. In general, in LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and AGWs. As such, the radio access network (RAN) of an LTE system has what is sometimes referred to as a “flat” architecture including radio base station nodes without reporting to radio network controller (RNC) nodes.
Transmission and reception from a node, e.g., a radio terminal like a UE in a cellular system such as LTE, can be multiplexed in the frequency domain or in the time domain (or combinations thereof). In Frequency Division Duplex (FDD), downlink (DL) and uplink (UL) transmission take place in different, sufficiently separated, frequency bands. In Time Division Duplex (TDD), DL and UL transmission take place in different, non-overlapping time slots. Thus, TDD can operate in unpaired frequency spectrum, whereas FDD generally requires paired frequency spectrum.
Typically, a transmitted signal in a radio communication system is organized in some form of frame structure, or frame configuration. For example, LTE generally uses ten equally sized subframes 0-9 of length 1 ms per radio frame as illustrated in FIG. 1. In case of TDD as shown in FIG. 1, there is generally only a single carrier frequency, and UL and DL transmissions are separated in time. Because the same carrier frequency is used for uplink and downlink transmission, both the base station and the UEs need to switch from transmission to reception and vice versa. An important aspect of a TDD system is to provide a sufficiently large guard time where neither DL nor UL transmissions occur in order to avoid interference between UL and DL transmissions. For LTE, special subframes (e.g., subframe #1 and, in some cases, subframe #6) provide this guard time. A TDD special subframe is generally split into three parts: a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). The remaining subframes are either allocated to UL or DL transmission. An example UL and DL configuration is shown in Table 1 below. Also, an exemplary special subframe configuration is shown in Table 2 below.
TABLE 1Exemplary UL and DL configuration in TDD.Uplink-Downlink-down-to-UplinklinkSwitch-configura-pointSubframe numbertionperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
TABLE 2Example configuration of special subframe.Normal cyclic prefixin downlinkExtended cyclic prefixUpPTSin downlinkNormalExtendedUpPTSSpecialcycliccyclicNormalExtendedsubframeprefixprefixcyclic prefixcyclic prefixconfigurationDwPTSin uplinkin uplinkDwPTSin uplinkin uplink0 6592 · Ts2192 · Ts2560 · Ts 7680 · Ts2192 · Ts2560 · Ts119760 · Ts20480 · Ts221952 · Ts23040 · Ts324144 · Ts25600 · Ts426336 · Ts 7680 · Ts4384 · Ts5120 · Ts5 6592 · Ts4384 · Ts5120 · Ts20480 · Ts619760 · Ts23040 · Ts721952 · Ts———824144 · Ts———
TDD allows for different asymmetries in terms of the amount of resources allocated for UL and DL transmission, respectively, by means of different DL/UL configurations. In LTE, there are seven different configurations, see FIG. 2. Generally speaking, to avoid significant interference between DL and UL transmissions between different radio cells, neighboring radio cells should have the same DL/UL configuration. Otherwise, UL transmission in one radio cell may interfere with DL transmission in the neighboring radio cell (and vice versa). As a result, the DL/UL asymmetry generally does not vary between radio cells. The DL/UL asymmetry configuration is signaled, i.e. communicated, as part of the system information and can remain fixed for a long time.
Consequently, the TDD networks generally use a fixed frame configuration where some subframes are UL and some are DL. This may prevent or at least limit the flexibility to adopt the UL and/or DL resource asymmetry to varying radio traffic situations. This may be particularly challenging in heterogeneous networks, e.g. a network comprising multiple pico cells and/or a mixture of multiple macro cells and/or pico cells. In order to provide a more flexible TDD configuration, so-called Dynamic TDD (also sometimes referred to as Flexible TDD) has therefore been introduced. Dynamic TDD provides the ability of a subframe to be configured as “flexible” subframe. As a result, some subframes can be configured dynamically as either for UL transmission or for DL transmission. The subframes can for example be configured as either for UL transmission or DL transmission depending on e.g. the radio traffic situation in a cell. Accordingly, Dynamic TDD can be expected to achieve promising performance improvement in TDD systems when there is a potential load imbalance between UL and DL. However, the benefits achieved by Dynamic TDD could potentially be counteracted or limited by inter-cell interference. Inter-cell interference may become a particular challenge when designing and/or deploying heterogeneous networks and has therefore been discussed in 3GPP. For example, Section 8 in the 3GPP Technical Report 3GPP TR 36.828, “Evolved Universal Terrestrial Radio Access (E-UTRA); Further enhancements to LTE Time Division Duplex (TDD) for Downlink-Uplink (DL-UL) interference management and traffic adaption”, v.11.0.0, identifies four different interference schemes that may cause additional complexity in radio communication networks using LTE TDD, e.g. in the eNBs or in the UEs or in both eNBs and UEs. Consequently, in heterogeneous networks it may become important to mitigate, alleviate or reduce the effects of interference.